- Impression when undercuts are present
- Partial denture with clasps
- Used even in cases of excess salivation
- Preliminary Impressions for complete denture
- Duplicating Models
- Mouth Guards
Ester salts of alginic acid Sodium or Potassium or Triethanlolamine aginate – 15%
Dissolves in water and reacts with calcium ions
Calcium Sulphate (Reactor) – 16%
Reacts with alginate (Na or K) and forms insoluble calcium alginate
Sodium Phosphate (Retarder) – 2%
Reacts preferentially with calcium sulphate to reduce the rate of reaction
Zinc Oxide – 4%
Potassium titanium fluoride – 3%
Diatomaceous earth – 60%
Coats powder particles in dust-free alginates
Colouring & Flavouring agent
It is a chemical reaction. Insoluble calcium alginate gel is formed when soluble alginate reacts with calcium sulphate.
To delay this reaction to occur in patients mouth, a retarder is added e.g. Na or K phosphate, oxalate or carbonate. By adding the retarder, calcium sulphate reacts with retarder first to increase the working time. The formed product may be:
- Tetrasodium pyrophosphate
- Sodium Tripolyphosphate
- Trisodium phosphate
2 reactions occur during setting:
Na3PO4 + 3CaSO4 -----→ Ca3(PO)4 + 3Na2SO4
Sodium alginate + CaSO4 + H2O ---→ Ca alginate + Na2SO4
Sodium phosphate reacts with calcium Sulphate give working time for the mix.
After Sodium Phosphate has reacted, Calcium Sulphate reacts with sodium alginate to form insoluble calcium alginate which is a gel when combined with water.
To control the gelation time,
Colder the water, longer is the gelation time
Warmer the water, shorter is the gelation time.
If the water/powder ratio is altered, the properties of alginate are hampered.
Clean flexible rubber bowl, spatula, Figure of 8. Swapping alginate against the wall of rubber bowl, make sure the mix is smooth and homogenous.
Figure of 8 is used because:
Promotes dissolution by mixing un-dissolved algin with dissolved algin
Remove most air bubbles
Fast set alginate – 45 seconds
Normal set alginate – 60 seconds
Over-mixing – Reduced working time, Reduced final strength due to destruction of gel fibrin.
Under-mixing – Reduced strength, Inadequate wetting, lack of homogeneity poor recording due to grainy mix
Fast set alginate – 1.15 minutes
Normal set alginate – 2 minutes
Properties of Alginate Hydrocolloid Impression Materials
According to ADA specification 18:
Fast Setting: 1.2 to 2 minutes
Normal setting: 2 to 4.5 minutes
Ranges from 1 to 5 minutes
For fast setting: 1 to 2 minutes
Normal Setting: 2 to 4.5 minutes
14% stress of 1000g/cm2. Lower W/P ratio results in lower flexibility.
Compressive: 5000 to 8000 g/cm2
Tear: 350 to 700 g/cm2
Due to airborne silica particles on fluffing of the alginate packet, some manufacturers are now supplying dust-free alginates.
Alginate Impressions loses water by evaporation and shrinks (Syneresis) and swells when kept in water (Imbibition). Hence, cast must be immediately poured after impression taking. Keeping in humid atmosphere of 100% relative humidity, least dimensional changes can be expected.
Words to remember:
Imbibition: Gain of water by immersion in water.
Syneresis: Loss of water by evaporation or exuding fluid
Good surface detail even in excess saliva
Easy to mix and manipulate
Minimum requirement of armamentarium
Accuracy if handled properly
Comfortable to patient
Flexibility of set impression
Cannot be corrected
Tissue compression is not achieved
Inaccurate for crown and bridge impressions
Distortion may occur easily
Metal dyes cannot be prepared.